1. Field of Application
This invention relates to safety brake mechanisms for vertical lifts; and more particularly, to safety brake mechanisms for cable hoist type vertical lifts.
2. Description of the Prior Art
Vertical lifts include all types of devices for moving between various levels along a vertical path. Freight elevators, people elevators, and the lifting carriages of stacker cranes and stacker-retriever type devices are merely examples of such equipment.
A considerable number of available vertical lifts are moved from one level to another by a power driven winch or hoist which utilizes a flexible member to raise, lower, and maintain the position of the lift. The flexible member, which can be a chain, cable or the like, extends between the power drive and the lifting platform; and in many instances the combination of drive and flexible member not only function to move the lift between its various levels, but are the only means to keep the lift so positioned. In such lifts, the lifting cage or platform is, in essence, hanging in space from the flexible member while the lift is being raised or is in raised positions above the ground level. Should the chain or cable, or their connecting members or drives, on such vertical lifts fail, then the lifting platform will plummet or free-fall to the ground level. People can be hurt or killed, and materials being carried may be damaged or destroyed.
Some available vertical lifts incorporate free-fall prevention devices, or brakes, to arrest or slow down what would otherwise be an unrestricted dropping of the vertical lift should a failure occur. However, those lifts which merely rely upon counterweighting to prevent free-fall may be found lacking since fixed-weight counterweights can prove to be ineffective for the job if the load weight is inadvertently increased, or will consume drive power if the load should be abnormally light or when the lift is moving while empty. Alternatively, the available lifts which utilize variable weight counterweighting require relatively complex mechanisms for adding and dropping off weights.
Some available vertical lifts, such as the one shown in U.S. Pat. No. 3,661,280, interpose a centrifugal clutch between the lift platform and the counterweights, but such constructions are relatively costly and complex; especially when they also provide mechanisms to mount a portion of the vertical lift for separation from the rest of the lift upon cable or other failure. Other available vertical lifts, such as shown in U.S. Pat. No. 3,606,039 rotate arcuate cam surfaces against a guide rail to thus provide a mechanical brake triggered into operation in response to a slack condition in the lifting cable for the elevator of a stacker crane. But such braking devices are also relatively complex in construction and operation and the cam shaped arcuate braking surfaces require relatively large spring forces to develop braking from what is essentially line contact with the guide rails. Such relatively large spring forces must, in turn, be overcome when the brake is to be inactive.